Generally, the admissions board won't review your packet until it's 100% complete. There have been cases when LOAs have been provided before the BGO interview. Also, if the BGO is delinquent in getting it done, eventually USNA will consider your packet before your interview. But, you should NOT assume your record will be considered before your interview is done. So, if that's the only thing that is missing from your packet, try to get it done soon.